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Activity 3: Gravity & Cohesiveness

WHAT DO YOU SUPPOSE IS THE "ANGLE OF REPOSE"?

Summary:

When you can understand how different rock sizes and shapes react with each other, then you can understand why some landforms have a certain shape. The following activity will help to explain the differences in hillside shapes.

Instructional Method:

Experiment

Goal:

To explain how gravity can be overcome by compactness and size.

Objectives:

Students will be able to:

Measure angles using a protractor

Determine the angle of repose for several different types of materials and a mixture of those materials

Explain the effect of rock shape, rock texture and moisture content on the angle of repose

The angle of repose is the angle at which loose rock is stable on a hillslope. The greater the angle of repose, the steeper the hillslope. Many factors attribute to the stability of different rock types including moisture, compaction, rock size, particle shape and rock texture. The angle of rock stability varies depending on rock type. One of the factors that can ensure a steeper hillslope and higher angle of repose is friction. The more surface friction rocks have the more stable they are.

The introduction of moisture in a hillslope environment can act like glue sticking the rock pieces together. If too much moisture is introduced it can push the particles away causing them to flow. A good example of this is a sand castle. Using a little water in the sand will allow it to stick and form walls. Too much water will turn the castle into mud and flow all over the beach.

Packing material down can increase stability of particles in a hillslope. This is called compaction. If rock pieces are loosely piled, they are not stable. Compacted rocks are closer together touching more rocks around them. They have more surface friction, which keeps them in place. If the material is somehow ordered (i.e., like a rock wall) as well as compacted, it will also add to its stability and increase the angle of repose, resulting in a nearly vertical hillslope.

Rock shape has an affect on the hill slope's stability and angle of repose. Flatter pieces can sometimes increase the hill slope stability depending on their orientation. If they are stacked like bricks, flat sides touching and nearly level, the angle of repose is very high. If they are slightly oriented down hill, the angle of repose decreases. A hillside of round rocks is not as stable as flat-stacked rocks. Round rocks can act like marbles rolling slowly down hill.

Rock texture will also vary the angle of repose. Smooth rocks do not have much friction and slide on each other easily. They have a low angle of repose. Rougher rocks have higher friction interactions with surrounding rock and will hold to each other, resulting in a steeper hillslope.

Rock size can play a factor in the hill slope's stability. Larger rocks place pressure on the rocks below them. Many times their weight keeps them in place for a long time, because it takes a lot of outside force to get them moving. Sometimes a rock's weight causes it to slide or roll downhill. Even though larger rocks may be stable, gravity will eventually pull them down.

The angle of repose for large rocks is high, but only for a relatively short time. Smaller rock sizes are easily affected by outside forces. Their angle of stability decreases because of the ease it takes to move the pieces. Mixing rock sizes, small and large, increases the angle of repose. Multiple sizes allow for more surface interaction and more friction between rocks. Friction increases the angle of repose.

Many National Parks were established to preserve natural beauty of a landscape. In many locations those landscapes are unstable, they change constantly by the processes of erosion. Looking specifically at the National Parks in and around Utah, you will find landscapes of great beauty speckled with large destructive landslides. These landslides are the result of unstable hill slopes. In nature, landslides are common and accepted. When hill slopes slide and cover manmade objects such as roads and buildings, the natural process is not always appreciated.

In the mid 1990's Zion National Park experienced a large landslide that dammed the Virgin River and covered the only road up the main canyon. At Northern Cascades National Park landslides due to unstable hill slopes slid into a valley at different locations, forming small dams along a stream. Beavers took advantage of the dams and reinforced them.

The following activity allows students to experiment with different variations to determine which rock mixtures allow for a higher angle of repose.

Instructional Procedures:

If using a stand clamp, attach the ruler so that it is parallel to the table, so the first measure mark is away from the stand. You should be able to adjust the height of the ruler easily, keeping it parallel to the table.

Place one of the cans on a level table and fill it with sand. Ask the students to predict what would happen if we lifted the can straight up. What would the pile of sand look like?

Pull the can up and have the students describe what the pile looks like. Explain that the side of the pile falls at an angle. We can measure that angle with our protractor. Slide the ruler down the stand so that it is parallel to the table but just touching the top of the pile of sand. (Do this carefully so as not to disturb the pile). Tighten the ruler so that it won't slide further if you let go.

Slide the whole stand away from the pile, and hold the protractor up to the ruler, curved side down.

Look through the protractor at the pile of sand, with the center of the flat side of the protractor aligned with the highest point on the pile. By keeping the protractor next to the ruler, we can ensure that the angle we are measuring is based off of horizontal. Estimate the angle made between the slope of the pile and horizontal, and note it on a piece of paper or the provided student page.

Fill another coffee can with sand, ask the students what the angle of this pile of sand will be: steeper, more gradual or equal. Repeat experiment to find out.

Repeat this experiment at least three times with the dry sand to determine the average angle of repose for sand.

Why does dry sand always show approximately the same angle? If you add water to the sand what happens to the angle? What if you add more water? Which material is the strongest and has the steepest angle? What has the weakest structure and the slightest angle? Did shape have anything to do with the angle? What about size, did heavier pieces hold up better? Did you test anything that had no angle at all? Why do some materials have steeper angles of repose than others, what are some factors that you observed? How do we see angle of repose in nature? (Use photos from National Park Service sites to show talus slopes, mountainsides, sand dunes, etc.)

Variation:

Large groups may want to break into stations of 5 students per station. Run through the first three experiments with dry sand as a class and then break out into stations, each station testing a different variation: silt vs. gravel, dry vs. wet, compacted dry vs. compacted wet, etc. The variations are endless; allow the students to develop their own experiments and try them out.